An early model of a laser-propelled lightcraft. Credit: Rensselaer Polytechnic Institute

One topic being
examined is beam-energy propulsion–using a beam of energy directed at a
spacecraft either to heat up its propellant or to deliver electricity to its
engine. By removing the energy source from the rocket itself, beam-energy
propulsion has the potential to make launching spacecraft cheaper and more
reliable.

In conventional
chemical propulsion, massive amounts of energy are stored in a rocket’s fuel,
which makes up a significant amount of its weight. In addition, chemical
systems are heated to temperatures above the melting point of some materials in
the rocket itself, says Alexander Bruccoleri, a researcher in the aeronautics
and astronautics department at MIT, who recently received his master’s from the
Space Propulsion Lab. Bruccoleri presented a paper at the conference on August
3 on a comparison metric he invented to test beam-energy systems.

Beam energy was
dreamt up in the late 1970s by NASA Ames Research Center and the California
Institute of Technology. “The idea was to use lasers as a heat
exchanger–take the energy and make a hot fluid that can expand out of the
nozzle,” Bruccoleri says. Now researchers are exploring ground-based
lasers systems that heat fuels such as hydrogen to a temperature that is easier
to manage. “The hydrogen molecules can be accelerated twice as fast as
water molecules with the same temperature, providing better exhaust
velocity–the thrust you get for the rate at which you are burning the
propellant,” says Bruccoleri. Using light as an external power source can
alleviate the weight and mass of having an onboard system, leaving room for
scientific payloads, for example, and it provides more propulsive power.

Leik Myrabo, an associate professor of mechanical,
aerospace and nuclear engineering a Rensselaer Polytechnic Institute in Troy, NY,
says the last three to five years have brought these systems closer to reality because
energy-beaming technology like laser beams and millimeter wavelength systems
have dropped in cost. Myrabo, founder of Lightcraft
Technologies, has demonstrated that one can propel a small “lightcraft” 71
meters in the air by using pulses of light that heat the propellant. He
currently has a five-year grant from the US Air Force to explore laser
propulsion to launch satellites for extremely low cost at high reliability, and
is conducting tests in Brazil in collaboration with that country’s air force.

While Myrabo says
that such systems could be a reality in 5 to 10 years, others are skeptical.
Kevin Johnson, a space exploration and spacecraft propulsion manager at Lockheed Martin Space
Systems in Denver, for example, expresses concern about the potential for atmospheric
interference with the beam. Greg McAllister, a senior staff propulsion engineer
also at Lockheed Martin, agrees and says that an energy source powerful enough
to propel a rocket could also burn it up. (McAllister is presenting a paper at
the conference on testing the pulse throttle thrusters used for the Mars
Phoenix mission.)

Johnson says that
while the system could generate enough power from a ground-based station and
reduce costs, it is “20-plus years” from being feasible.

A test flight of a lightcraft using pulses of light conducted by Myrabo in 2000 at White Sands Missile Range in New Mexico. Credit: RPI